From the Guidelines
CTA is generally adequate for evaluating posterior circulation strokes, but it has some limitations, and its accuracy can be affected by factors such as beam-hardening artifacts from the skull base and the presence of atheromatous calcifications. While CTA provides excellent visualization of large vessel occlusions and stenosis in the vertebrobasilar system, it may miss smaller vessel abnormalities or subtle changes. For comprehensive evaluation of posterior circulation strokes, CTA should ideally be combined with non-contrast CT and CT perfusion imaging when available.
Key Points
- CTA's sensitivity and specificity for the detection of intracranial occlusions ranges between 92% and 100% and between 82% and 100%, respectively, with a positive predictive value of 91% to 100% 1.
- MRI with diffusion-weighted imaging and MR angiography remains the gold standard for posterior fossa stroke detection due to its superior sensitivity for small infarcts and reduced bone artifact compared to CT.
- CTA's advantages include widespread availability, rapid acquisition time, and ability to detect large vessel occlusions that might benefit from thrombectomy.
- However, beam-hardening artifacts from the skull base can sometimes obscure visualization of posterior circulation vessels.
- In emergency settings where rapid decision-making is crucial and MRI is unavailable, CTA provides adequate information for initial management decisions in posterior circulation strokes, particularly for identifying candidates for endovascular intervention.
Considerations
- The presence of atheromatous calcifications can affect the accuracy of CTA, with some studies reporting a 13% false-positive rate for occlusion when heavy atheromatous calcifications are present 1.
- Appropriate window and level adjustments can help account for the blooming artifacts associated with heavy calcific plaque, improving the accuracy of CTA for stenosis quantification 1.
- The use of low-pitch or delayed CTA may be beneficial in certain cases, such as when DSA shows a posterior circulation vessel to be occluded 1.
From the Research
CT Angiography for Posterior Stroke
- CT angiography (CTA) is a common follow-up study after noncontrast head CT to identify intracranial large vessel occlusions and cervical carotid or vertebral artery disease 2.
- CTA is highly sensitive and can improve accuracy of patient selection for endovascular therapy through delineations of ischemic core 2.
- However, the diagnostic value of CTA in suspected posterior circulation stroke is uncertain, and whole brain volume perfusion is not yet in widespread use 3.
Comparison with Other Imaging Modalities
- Computed tomography perfusion (CTP) has a high diagnostic value in the detection of acute ischemic stroke in the anterior circulation, but its value in posterior circulation stroke is less certain 3.
- CTP detected significantly more ischemic lesions, especially in the cerebellum, posterior cerebral artery territory, and thalami, compared to CTA and noncontrast CT alone 3.
- Multiphase CT angiography compares well with CTP in selecting patients for endovascular thrombectomy in the late time window 4.
Treatment of Posterior Circulation Stroke
- The effectiveness of acute revascularization of posterior circulation strokes remains largely unproven, but thrombolysis seems to have similar benefits and lower hemorrhage risks than in the anterior circulation 5.
- Thrombectomy benefits strokes with basilar artery occlusion, but its effect on other posterior occlusion sites remains uncertain 5.
- Secondary prevention of posterior circulation strokes includes aggressive treatment of cerebrovascular risk factors and short-term dual anti-platelet therapy 5.